Algal Studies, East Fork Carson River
Excessive amounts of algae are found during summer months in a 2 mile reach of the East Fork Carson River within Carson Valley, Nevada. Past studies have focused on factors in the Carson River that are associated with eutrophication including stream water temperature, river substrate, nutrient concentrations, and diel (24 hour period) dissolved oxygen (DO) cycles. Summer dissolved nitrate concentrations in this reach are higher compared to other sites on the Carson River, which may have contributed to the high algae growth. previous studies indicate that groundwater discharges to the East Fork Carson River in this reach during the summer. How groundwater discharging to the river affects the nutrient concentrations in the river and contributes to algal growth has not been investigated.
Data for this pilot study will be collected during the 2010 summer field season in a 2 mile study reach of the East Fork Carson River. USGS, NDEP, and CWSD will work collaboratively to
- determine the nitrate concentrations and estimate loads of groundwater discharging to the study reach,
- estimate the instantaneous nutrient loads in the river in the study reach, and
- determine the algal species and level of eutrophication in the East Fork Carson River.
Specific tasks include
- installing temporary shallow wells and temperature probes,
- sampling wells and analyzing water using ion selective probes,
- sampling wells over 24 hour period,
- sampling stream and algae and estimating instantaneous stream nitrate loads,
- determining aquifer properties, and
- estimating nitrogen loading to the river from groundwater.
NDEP has observed eutrophication problems, defined by NDEP as excessive amounts of algal growth and/or low dissolved oxygen concentrations, in a 2 mile reach of the East Fork Carson River within the Carson Valley between Highway 88 and Muller Lane. NDEP found that stream median summer nitrate plus nitrite concentrations in the study reach were higher (0.10 mg/L as nitrogen) compared to other sites on the Carson River (the highest median value upstream was 0.03 mg/L and the highest median value downstream was 0.03 mg/L (Pahl, 2007). USGS (Maurer and others, 2009) found that total nitrate plus nitrite concentrations in the study reach were higher compared to other sites on the Carson River (1.8 mg/L as nitrogen compared to 0.4 mg/L). The higher concentrations may contribute to the high algae growth in this reach.
NDEp recommended that additional study is warranted in this reach of the East Fork Carson River to understand the source of the eutrophication problem and to document the conditions that cause the excessive algal growth. Excessive algal growth is unsightly to residents and visitors to Carson Valley and a nuisance (due to clogging) to agricultural drains and water distribution systems. Algal growth is also an indication that the ecology of the river is out of balance so that other potential issues may arise in higher parts of the river food chain. Pahl (2008) found that in the proposed study reach, the water temperature varied substantially (increases and decreases) as one moves downstream, whereas farther upstream from the study reach the temperatures increased steadily. The reasons given for the temperature variability include several flow diversions, and areas of groundwater discharges that could be a potential cooling source.
Concentrations of nitrate from groundwater discharging to the Carson River may add to the excessive algal growth in summer months. The highest nitrate concentrations in groundwater up gradient of the study reach is nearly twice the concentration in groundwater down gradient (20 mg/L compared to 11 mg/L). Determining the contribution of nitrogen to the Carson River from groundwater during summer months will allow water managers to develop strategies for mitigating these contributions.
NDEP and CWSD are interested in improved characterization (temporal and spatial variability) of the level of eutrophication in the study reach. previous work by NDEP has estimated algae levels in terms of "percent cover of substrate" but more quantitative measures are desired. Under this study, data will be collected which quantify algal biomass density in terms of chlorophyll-a and ash free dry weight, in addition to qualitative estimates of percent cover by benthic algae. These values can then be compared to thresholds in the literature to better describe the level of eutrophication, and its variability throughout the study reach. In addition, algae species occurring in the area will be identified to determine whether cyanobacteria (nitrogen-fixing algae) are present, and their contribution to the overall algal biomass. Knowing the type of algae present will help to estimate their nutrient requirements, and this will lead to improved understanding of the nutrient-algae dynamics in the study reach.
Start Date: 2010
End Date: 2011
Cooperator: Carson Water Subconservancy District
USGS Nevada Water Science Center
2730 N. Deer Run Rd.
Carson City, NV 89701
phone: (775) 887-7644
CWSD: Carson Water Subconservancy District
mg/L: milligrams per liter
NDEp: Nevada Division of Environmental protection